Editing IPCC:AR6/WGII/Cross-Chapter-Paper-3 (section)

==== CCP3.3.1.2 Rainfall, Evaporation and Drought ====

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Drylands are highly sensitive to changes in precipitation and evapotranspiration. Potential evapotranspiration (PET) is projected to increase in all regions globally, under all RCPs, as a result of increasing temperatures and surface water vapour deficit ( [[#Mirzabaev--2019|Mirzabaev et al., 2019]] ). Simulations based on coupled land surface, energy, and water and vegetation models in the Central Sahel showed a strong response of the water budget. Under +2°C and +4°C warming scenarios, decreased evapotranspiration, runoff and drainage were found for all scenarios, except those with the highest precipitation ( [[#Léauthaud--2015|Léauthaud et al., 2015]] ).

Globally, soil moisture declined over the 20th century ( [[#Gu--2019|Gu et al., 2019]] ), a trend that is projected to continue under all emissions scenarios (WGI). Projected drier soils can further amplify aridity through feedbacks with land surface temperature, relative humidity and precipitation ( [[#Berg--2016|Berg et al., 2016]] ).

Drought conditions (frequency, severity and duration) are expected to substantially worsen in global drylands, driven by a higher saturation threshold and more intense and frequent dry spells under rising temperatures ( [[#Liu--2019|Liu et al., 2019]] a; 2019b). In a +1.5°C world, historical 50-year droughts (based on the Standardised Precipitation-Evapotranspiration Index (SPEI)) could occur twice as frequently across 58% of global landmasses relative to the 1976–2005 period, an area that increases to 67% under 2°C warming ( [[#Gu--2020|Gu et al., 2020]] ). Multi-year drought events of magnitudes exceeding historical baselines will increase by 2050 in countries with drylands including Australia, Brazil, Spain, Portugal and the USA (Jenkins and Warren, 2015). The magnitude of drought stress in different regions differs depending on the metric used. Projections based on the PDSI suggest drought stress will increase by more than 70% globally, while a substantially lower estimate of 37% is found when precipitation minus evapotranspiration (P – E) is used ( [[#Swann--2016|Swann et al., 2016]] ). However, the two metrics agree on increasing drought stress in regions with more robust decreases in precipitation, such as southern North America, northeastern South America ( [[IPCC:Wg2:Chapter:Chapter-12#12.3.1.1|Section 12.3.1.1]] ) and southern Europe ( [[IPCC:Wg2:Chapter:Chapter-13#13.1.3|Section 13.1.3]] ; [[#Swann--2016|Swann et al., 2016]] ).

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